1. Field of the Invention
The present invention relates to a signal generator. It is particularly concerned with a signal generator for applying a signal to a conductive cable or pipe, particularly but not exclusively, to such a cable or pipe buried underground.
2. Summary of the Prior Art
It is well known to determine the location of an underground cable or pipe by detecting the magnetic field generated by an alternating current carried by such a cable or pipe. Thus, for example, EP-A-0825456 discloses an arrangement for detecting the condition of a buried metal pipeline in which an alternating signal is applied to the pipe and detected at the surface by a suitable detector.
In order for such a magnetic field to be detected, it is usually necessary for a suitable alternating signal to be applied to the pipe or cable, and a signal generator is used to do this. The output of the generator is coupled to the pipe or cable using one of three methods: by making a direct connection, by using an induction coil which induces a current in the conductor, and by using a toroidal clamp around the pipe or cable which induces current by a transformer effect.
The ground impedance associated with the cable or pipe varies greatly depending on site conditions and so the magnitude of the current in the cable or pipe is frequency dependent. The use of higher frequencies enables more signal current to be driven along the cable or pipe but that signal decreases more rapidly with distance along the conductor or pipe from the generator than a low frequency signal. The optimum operating frequency thus depends on the length of the cable or pipe and the ground impedance, how the latter is distributed along the cable or pipe, and how far the operator wishes to trace the cable or pipe.
Therefore, the generator needs to be capable of generating a range of frequencies. Moreover, as a result of the wide range of ground impedances, the generator also needs to be capable of operating over a wide load impedance range, from short-circuit to open-circuit, including any combination of resistive, capacitive and inductive reactances. It is also usually necessary for the generator to be portable.
Generators are known which employ a waveform synthesiser to produce a sinusoidal waveform that is applied to an input a class AB amplifier. The frequency generated by the synthesiser is controlled by a microprocessor, which also controls a variable power supply to the synthesiser, which power supply determines the amplitude of the waveform generated. Whilst it is possible to make such an arrangement compact, and relatively inexpensive, it has the disadvantage that its power efficiency is low, typically of the order of 50% Since the generator, if portable, is usually powered by a battery, this produces significant drain on battery life, and also has the disadvantage of making the generator produce more heat than is desirable.
The present invention seeks to provide a generator suitable e.g. for applying a signal to a cable or pipe, and at its most general proposes that the signal is generated by a delta-sigma modulator or oscillator. The output of that modulator or oscillator may then be passed via a power switching stage and an output filter to generate the output of the generator. Is has been found that the use of such a delta-sigma oscillator or modulator enables greater energy efficiency to be achieved in the power switching stage.
The present invention has four aspects, each comprising a generator and a method of generating signals using such a generator. In the first aspect, a waveform synthesiser is used to generate a desired waveform, which is input to a delta-sigma modulator. In the second aspect, the separate waveform generator may be omitted, by the use of a delta-sigma oscillator to replace the waveform generator and the delta-sigma modulator. Each of these two aspects are xe2x80x9creal-timexe2x80x9d, in that they assume that the signal is being generated at the time at which it is output. However, in the third aspect, a signal is generated using either the combination of waveform generator and delta-sigma modulator, or a delta-sigma oscillator, and a suitable sample of that waveform is stored in a memory of the generator for subsequent retrieval at the time the output is to be generated.
In the fourth aspect of the invention, a delta-sigma modulator or oscillator is used to generate a control program for controlling the power switching stage so that control program is then stored in a memory of the generator for subsequent retrieval at the time the output is to be generated.
In each of the above aspects, the delta-sigma modulator and the waveform generator (if any) can be a digital circuit.
Delta-sigma modulators and oscillators (hereinafter xcex94-xcexa3 modulators and oscillators) are known devices in which an input is passed to a quantizer via an integrator and the quantized output feeds back to subtract from the input signal. This feedback causes the average value of the quantized signal to track the average value of the input signal. Any persistent difference accumulates in the integrator and eventually corrects itself.
Thus, the xcex94-xcexa3 system can be considered to be formed by the series connection of at least one accumulator and a quantizer, each of which involves a feedback signal. xcex94-xcexa3 modulators may have multiple accumulator stages, and even possibly multiple quantizer stages. The number of stages of accumulation are referred to as the xe2x80x9cordersxe2x80x9d of the xcex94-xcexa3 modulator, when there is a single quantizer stage, so that such a modulator with one accumulation stage is referred to as a first-order xcex94-xcexa3 modulator, one with two such accumulation stages is referred to as a second-order xcex94-xcexa3 modulator, etc.
In such an arrangement, in which the signal generated by a xcex94-xcexa3 modulator or oscillator is passed via a power switching stage and output filtered to generate the output to a load, the control of the generator output can be achieved in several different ways. The generator output current, power or voltage can be fed back to the xcex94-xcexa3 modulator or oscillator, to control it, or the output current, power or voltage can be fed back to the power supply controlling the power switching to achieve a similar effect. It is also possible to control the output of the generator on the basis of the power supply current, power or voltage delivered to power the switching stage. Another possibility would be to use current, voltage or power regulation in the power supply and yet another possibility would be to control the amplitude of the waveform generator.